Engine out cold-start emissions generated before light-off of an exhaust system catalytic converter may contribute a large percentage of the total exhaust emissions. Various approaches may be used by engine control systems to expedite the attainment of the catalyst light-off temperature. For example, various combinations of valve overlap, fuel injection, and spark retard may be used to expedite catalyst warming.
One example approach, shown by Surnilla et al. in US 20140297162, leverages a split fuel injection to improve cold-start emissions and driveability. Therein, during a cold start, fuel is provided as a split fuel injection for a number of combustion events with a portion of fuel direct injected in an intake stroke, another portion of fuel direct injected in a compression stroke, and a remaining portion of fuel port injected in an exhaust stroke. In addition, spark timing is retarded. The split injection results in a stratified combustion that enables less fuel to be used while allowing for a more stable combustion as compared to a single fuel injection.
However, the inventors herein have recognized potential issues with such systems. As one example, use of split injection for an extended time may result in engine roughness. The roughness may be exacerbated as the engine warms up due to more fuel evaporating at the warmer temperature, resulting in rich misfires. The inventors have recognized that the roughness may be due to the fuel injection timing applied on the split injection being better suited for the cold engine at the cold-start, but not well suited for a partially warm engine or for a hot engine restart. On the other hand, if the split injection is replaced with a single homogeneous (e.g., intake stroke) injection to address the engine cold-start NVH issues, engine roughness may be experienced during engine idling. As such, this can lead to a misfire diagnostic code being erroneously set, causing false warranty issues.
In one example, the issues described above may be at least partly addressed by a method for an engine, comprising: starting an engine with fuel delivered as a split injection; and adjusting a fuel injection timing based on engine temperature, the injection timing advanced as the engine temperature increases. In this way, engine cold-start roughness can be reduced.
As one example, during an engine cold-start, fuel may be delivered to a cold engine as a split compression stroke direct injection for a number of combustion events since a first combustion event of the engine cold-start. Herein, for a given combustion cycle, a portion of the total fuel amount may be direct injected into the engine during an earlier portion of a compression stroke, and a remaining portion of the total fuel amount may be direct injected during a later portion of the same compression stroke. In addition, fuel may be delivered at an (average) injection timing that is closer to compression stroke TDC. Specifically, the fuel injection timing may be based on the engine coolant temperature at the engine start, before the engine starts rotating, and before a first combustion event has occurred in the engine. Then, as the engine is cranked and the engine temperature increases, while maintaining the split compression stroke direct injection, the average injection timing may be advanced from compression stroke TDC. In addition, while injection timing is advanced, spark timing may be retarded from MBT to expedite engine heating. After the engine has been sufficiently warmed (e.g., after a number of combustion events since the start), the injection timing may be retarded and fuel may be delivered as a single intake stroke direct injection.
In this way, by using a split compression injection during an engine start along with an injection timing that is adjusted based on engine temperature, engine start roughness issues can be reduced. The technical effect of advancing an average injection timing of an engine start split injection with rising engine coolant temperature is that engine smoothness is improved during both engine cold-starts and engine hot-starts. By delaying the transition to a single fuel injection, fuel economy is improved. When the injection timing is advanced during an engine cold-start, a fueling requirement of the cold engine and cold exhaust catalyst is reduced, improving cold-start emissions. By reducing the occurrence of false misfires, false warranty issues are also improved. Overall, engine start performance (for both engine cold-starts and hot starts) is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.